Indoor versus Outdoor Air - which is more polluted

One keeps seeing ads in newspapers and magazines about the noxious air that we are breathing indoors and how our indoor air is actually 10 times worse than outdoor air and hence we MUST buy an air purifier to breathe cleaner air.  Lets not get carried away with this marketing and try to understand if our indoor air really is as bad as it is made out to be.
To understand air quality, we are going to break it up into:.

• Particulate pollution – PM2.5, PM10
• Gaseous pollution – SOx, NOx, O3, CO2, VOC’s
• Microbiological pollution – Bacteria, Virus, Mold

Particulate pollution is one of the main pollutants in the air – this is extremely small particulate matter that stays suspended in the air, is invisible and we breathe it in all the time.  As per the WHO, PM2.5 (particles that are less than 2.5 micrometres in diameter) should be below 50 micrograms per cubic meter in air outdoors and less than 15 micrograms per cubic meter in air indoors. Our numbers in Delhi outdoors are way higher (10 to 30 times higher than recommended upper limits).  However, this pollutant, is either equal in indoor spaces as compared to outdoor spaces or is typically marginally lower by as much as 50%.  And this is the main pollutant that good air purifiers are actually effective at removing.  So if outside air is at 300 micrograms per cubic metre and we have 150 micrograms per cubic metre, and the WHO recommendation for indoor air is 15 micrograms per cubic metre, it is adequate to say that a good air purifier will help bring this PM2.5 down to acceptable levels.
Gases and Odour:  Some of the pollutants from this next category of pollutants are typically what are much higher indoors versus outdoors – sometimes as much as 10 times higher - they are CO2 and VOC’s.  SO2, NO2 and O3 are typically higher outdoors.  But do air purifiers really address CO2 and VOC’s?  No.  They do NOT.  No air purifier in the world today is capable of reducing or removing CO2.  Some of them reduce VOC’s marginally, but truly do NOT do much to these carcinogenic gases – like formaldehyde, benzene, acetone, etc.  Hence, one MUST air out their rooms in order to get rid of these toxic gases from the air.  So the main claim of indoor air being 10 times worse that outdoors is not even addressed adequately by an air purifier.
Microbiological pollutants:  These pollutants are typically marginally higher indoors versus outdoors. And a lot of technologies aim at removing these pollutants from the air – UV, negative ionizers, ozonisers, photo catalytic oxidation, etc.  However, one must be extremely cautious about these technologies, as they can produce Ozone as a by-product and end up hurting one in the long run as prolonged exposure to even mild levels of Ozone is very harmful for health.  Also, if one eliminates all the bacteria and virus in indoor spaces and makes it sterile, it may not be so good, as living in a sterile environment ALL the time can brings down immunity and one may fall ill if exposed to any bacteria, virus or mold.
But removing the first 2 pollutants (particulate matter and harmful gases) is actually good for us and improves our immunity.  If one thinks that by breathing air polluted with PM and gases one is becoming immune and stronger, then one should encourage their children to smoke a packet of cigarettes every day.  Now that would be something.
Removing PM2.5 and odor can actually help reduce stress on the lungs and help one breathe better and reduce the symptoms of wheezing and asthma.  Hence an air purifier may actually be a good thing – but NOT because our indoor air is 10 times worse than the outdoors.

Barun Aggarwal
Barun Aggarwal is an entrepreneur and MBA with multi-continent experience. He has actively engaged in campaigns around climate crisis issues relating to air quality, water conservation, and energy efficiency. Barun founded Breathe Easy Consultants to help improve indoor air quality and is actively engaged with The Climate Project Foundation (the Indian branch of The Climate Reality Project founded by Nobel Laureate Al Gore). 

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